A magnetic matrix display of the present invention is particularly although not exclusively useful in flat panel display applications such as television receivers and visual display units for computers, especially although not exclusively portable computers, personal organisers, communications equipment, and the like.
Conventional flat panel displays, such as liquid crystal display panels and field emission displays, are complicated to manufacture because they each involve a relatively high level of semiconductor fabrication, delicate materials, and high tolerances.
GB Patent Application 2304981 discloses a magnetic matrix display having a cathode for emitting electrons, a permanent magnet with a two dimensional array of channels extending between opposite poles of the magnet, the direction of magnetisation being from the surface facing the cathode to the opposing surface. The magnet generates, in each channel, a magnetic field for forming electrons from the cathode means into an electron beam. The display also has a screen for receiving an electron beam from each channel. The screen has a phosphor coating facing the side of the magnet remote from the cathode, the phosphor coating comprising a plurality of pixels each corresponding to a different channel.
In a colour magnetic matrix display, each of the corresponding phosphor pixels may be a group of phosphor elements, each group corresponding to a different channel and each group typically comprising a Red, a Green and a Blue phosphor element. There are first and second deflection anodes for sequentially addressing electron beams emerging from the channels to different ones of the phosphor elements thereby to produce a colour image on the screen. The first and second deflection anodes are arranged as a pair of combs. There is a significant capacitance between these combs, which means that for high speed applications such as high refresh rates or autostereoscopic displays, high current driver amplifiers must be used in place of low cost standard integrated circuit amplifiers.
There are grid electrodes disposed between the cathode and the magnet for controlling the flow of electrons from the cathode into each channel. These control grids comprise a first group of parallel control grid conductors (first grid) extending across the magnet surface in a column direction and a second group of parallel control grid conductors (second grid) extending across the magnet surface in a row direction so that each of the channels is situated at the intersection of a different combination of a row grid conductor and a column grid conductor. In operation, each of the first group of grid conductors are held at one of two fixed potentials, whilst each of the second group are driven to analog voltages that will determine the beam current which will flow.
There is a significant capacitance between the second grid conductors and the other electrodes of the display as well as to ground, which will act to slow the drive signal rise and fall time. There is also a mutual coupling capacitance between the first grid conductors and the second grid conductors, which will cause a transient on the second grid conductors when the first grid conductors is switched from one to the other of the two fixed potentials. Further there is a mutual coupling capacitance between adjacent conductors of the second grid conductors, which causes crosstalk between the channels associated with adjacent ones of the second grid conductors.
Additionally, the grid drive voltages for certain applications such as a display using a very low beam current or using a high beam current may be outside the range desirable in order to minimise the cost of the grid drivers. The second grid conductors, which are driven by digital to analog converters (DACs), should ideally be capable of being driven at CMOS compatible voltages. Too high a voltage leads to expensive drivers, such as those which are used in Plasma panels. Too low a voltage leads to excessive difficulty in controlling beam current due to sensitivity to electrical noise, DAC linearity and the like.
Co-pending GB Patent Application 9611469.9 discloses a sensor array plate consisting of metal strips across the path of the electron beam, separate from the normal grid structure. This provides real time active beam current sensing for each channel of a magnetic matrix display. Such a structure, having an array separate from the normal grid structure is complex and has many interconnections.
Co-pending GB Patent Application 9703807.9 discloses a magnet for a magnetic matrix display having an insulated plate located on the side facing the cathode, the surface of the flat insulated plate facing the cathode being at a predetermined distance from the control grid (first grid) and being perforated with one or more apertures for each of the one or more electron beams. Such an insulated plate and the control grids mentioned above require several processes to make and leave small gaps which are difficult to evacuate when the glass envelope is evacuated. Additionally, the grid connections must be brought out to the driver chips, located external to the glass envelope.
One of the requirements of a magnetic matrix display is to precisely align the apertures in the magnet with the phosphor stripes on the screen. If uniform column stripes are used the problem becomes one of horizontal alignment and rotational alignment. Alignment of the structure to less than the width of one phosphor stripe is possible with the use of optical alignment aids, such as, for example, by aiming a laser through preselected apertures whilst the magnet and screen are held and adjusted in a fixture. However, some residual adjustment may be necessary after construction of the magnetic matrix display, to give optimal colour purity. Co-pending GB Patent Application 9612345.0 discloses a method by which small horizontal adjustments may be made by introducing an offset voltage onto the deflection anodes. Co-pending GB Patent Application 9625235.8 discloses a method of electronic control of rotation, but such a method requires a more complex manufacturing process for the grid assembly.
Co-pending GB Patent Application 9706992.6 discloses a magnet for a magnetic matrix display wherein the magnet extends beyond the area occupied by the array of channels such that the field strength in the channels at the periphery of the array is substantially equal to the field strength in channels at the centre of the array. This extended area is not used for the display of information due to the non-linearity of any display in this area. However, the area does produce a non-display border around the active display area, which increases the size of the display.